As semiconductor devices become highly integrated and the thickness of gate insulating layers becomes smaller, new materials are being developed for the gate insulating layers. Silicon oxide (SiO2) layers may be used as gate insulating layers in today's semiconductor devices. Silicon oxides may be used as gate insulating layers because of their thermal stability, reliability, and ease of production. However, the dielectric constant of silicon oxide layers is about 3.9, which may pose some limitations on the ability to scale silicon oxide layers to different sized semiconductor devices. In particular, the leakage current of silicon oxide can greatly increase as the thickness of the silicon oxide layer decreases.
As an alternative to silicon oxide layers, high dielectric layers have been studied. High dielectric layers can be used in place of silicon oxide layers as gate insulating layers. When high dielectric layers are used as the gate insulating layer, leakage current can be reduced by making the high dielectric layer thicker than the silicon oxide layer while maintaining the same capacitance value. Some common substances that may be used in the formation of high dielectric layers include (Bax, Sr1-x) TiO2 (BST), TiO2, Ta2O5, Al2O3, ZrO2, Zr silicate, HfO2, or Hf silicate.
The use of high dielectric layers as gate insulating layers, however, may have some problems. For instance, if a BST layer, a TiO2 layer, or a Ta2O5 layer is directly deposited on a silicon substrate, interfacial characteristics with the silicon substrate may become poor and the leakage current may increase. In addition, interface trap charge densities may increase and the mobility may decrease. The use of aluminum oxide (Al2O3) as a high dielectric layer may provide a high thermal stability, however, aluminum oxide has a relatively low dielectric constant of about 11 and a threshold voltage that can be difficult to control.
It has also been suggested to use a zirconium oxide layer, a zirconium silicate layer, a hafnium oxide layer, or a hafnium silicate layer as high dielectric layers. Each of these compounds may have moderate thermal stability and mid level dielectric constants of about 12–25. However, zirconium oxide layers can react with polysilicon, therefore it may not be desirable to use zirconium oxide layers alone. Hafnium oxide layers can become thick and can be easily crystallized, resulting in increases in the leakage current through grain boundaries. In addition, the threshold voltages of devices including zirconium oxide layers and hafnium oxide layers can be difficult to control, making these compounds undesirable as high dielectric layers.